1. Field of the Invention
The present invention relates to a block copolymer comprised of vinyl aromatic polymer block and polybutadiene block, and the method of preparing the same, wherein polyisoprene block is present between the blocks of vinyl aromatic polymer and polybutadiene. More specifically, the present invention relates to a block copolymer comprised of vinyl aromatic polymer and polybutadiene with enhanced mechanical strength. By changing the relative contents of each polymer block, mechanical property of the block copolymer can be readily controlled.
2. Description of the Related Arts
Styrenic block copolymer composed of polymer blocks of vinyl aromatic monomer and conjugated diene has generally diblock or triblock structure and is regarded as a thermoplastic elastomer. The polystyrene-block-polybutadiene-block-polystyrene(SBS), where the conjugated diene is butadiene, is one of the common styrenic block copolymer(SBC). SBS has a triblock structure comprising the soft middle block of polybutadiene and the hard blocks of polystyrene at the both ends. Such a combination of polymer blocks and structural characteristics provide useful properties applicable to a wide range of area as a thermoplastic elastomer. Polystyrene block and polybutadiene block of SBS do not mix each other and constitute a micro phase separation. Such a phenomenon differs from the phase separation of the blend of two different homo polymer, since polystyrene block and polybutadiene block in SBS are connected by covalent bond. The block sequence that the middle block is elastomeric polymer and the both ends are hard polymer blocks provides a phase separation structure as the hard glassy domains are connected by elastic polymer chains consisting matrix phase. Thus the polymer material has both of thermoplastic and elastomeric character, the former being provided by the glassy phase polymers with high mechanical strength and the latter from soft polymer block with elasticity which is able to store the mechanical stress in the polymer chains. The dual property of SBS as a thermoplastic elastomer finds a wide area of application in asphalt modification, compounding, shoe parts and adhesive applications. The ternary block structure obtained from polystyrene and polyisoprene, SIS (polystyrene-block-polyisoprene-block-polystyrene) is generally used for adhesives application.
Ternary block copolymers comprising styrene, butadiene and isoprene, with various structures were presented. In U.S. Pat. No. 3,692,874, the method of enhancing coupling efficiency by introducing butadiene block polymers at the ends of polyisoprene block is disclosed. In USP 5,292, 819 and USP 5,399, 627, a radial block copolymer represented by a formula (pS—pI—pB)nX, (n>2) is disclosed. For the purpose of facile coupling reaction to give radial structure, introduction of pB block less than 10 weight %, particularly less than 5 weight % is suggested. In the literature (JPS vol 36 1743–53, 1998), the relatively slow coupling reaction of anionic polymer with polyisoprenyl end using dichlorodimethylsilane as a coupling agent is reported. The coupling efficiency was 79% after 20 hours in benzene solvent. According to this publication, the substituted methyl group of isoprene imposes a larger steric hindrance than butadiene and is ascribed as the main factor resulting in the low efficiency in the coupling reaction. For this reason, only small amount of butadiene is added for end-capping of polyisoprenyl anion and the elastomeric entity of the copolymer is largely composed of polyisoprene. Accordingly, the obtained block copolymer has the properties of SIS, which is composed of polystyrene block and polyisoprene block.
There are examples of block copolymer containing polyisoprene block and polybutadiene block together to enhance heat stability. Generally, the polybutadiene undergoes cross-linking reaction at high temperature, which results in increase of viscosity. On the other hand, polyisoprene, when heated, tend to be less viscous due to bond scission process. Combination of polyisoprene block and polybutadiene block together in a single polymer cross cancels the viscosity change under heat aging condition and increase the heat stability.
In U.S. Pat. No. 4,120,915, disclosed is a block copolymer represented by S-D1-D2, which comprises of polymer blocks of styrene, conjugated diene and the second conjugated diene. Polymer blocks of conjugated diene comprising butadiene and isoprene are introduced to increase heat stability. However, this polymer has over 70% of styrene and the increase of physical property by combining conjugated diene blocks is not mentioned. The polymer has multi-branched structure induced by multi-functional coupling agent.
In U.S. Pat. Nos. 5,532,319 and 5,583,182, described is a ternary block copolymer represented by the general formula (S—B—I)nX, which has an improved heat resistance when incorporated in hot melt adhesive formulation.
Most of the prior arts, described above, suggest the addition of a little amount of butadiene at the ends of the vinyl aromatic-isoprene block copolymer to improve the efficiency of coupling reaction or to increase the heat resistance. But enhanced mechanical property of vinyl aromatic-butadiene block copolymer accompanied with combination of polysisoprene and polybutadiene was not described and control of the content of vinyl aromatic component without sacrifice of mechanical strength was not mentioned.
Commercial product of SBS generally has about 30 weight % of styrene content. If the styrene content is lower than this, mechanical strength is drastically affected. The content of styrene used in the SBS product is in the vicinity of limiting point for the polymer to maintain thermoplastic elastomer character derived from effective phase separation. However, styrene content in the block copolymer also affects its melt and solution viscosities, both of which are indicators of processability of the block copolymer as well as its mechanical properties such as tensile strength and hardness. Variations in such physical properties and contents of styrene are in demand depending on specific application.
In SIS where isoprene is used instead of butadiene, at 15 weight % of vinyl aromatic content, sufficient mechanical properties can be obtained due to the greater difference in solubility parameter of two components, compared with the vinyl aromatic-butadiene block copolymer. According to the thermodynamic theory, the phase separation of the block copolymer depends on the interaction parameter of the block components, the molecular weight of the block copolymer and the relative content of the components. (Hamley, I. W., The Physic of Block Copolymers; Oxford University Press; Oxford, U.K. 1998). The interaction parameter is determined by the difference of solubility parameter of the components of the block copolymer. These three factors simultaneously affect the over-all phase separation of block copolymer. Accordingly, in order to obtain effective phase separation and maintain the property of the thermoplastic elastomer when the relative content is changed, compensation by controlling other factors such as interaction parameter or the molecular weight is needed. The present invention, in this regard, provides a block copolymer of vinyl aromatic monomer, butadiene, and isoprene with various relative content of components without sacrificing physical properties as a thermoplastic elastomer, derived under in depth consideration of the thermodynamic parameters which affect phase separation.